1. Field of the Invention
This invention concerns a semiconductor device used for mobile communication apparatus such as cellulars operation in a microwave band of 500 MHz to 2.5 GHz and, more in particular, it relates to a technique which is effective to high frequency powers amplifier of power-amplifying high frequency signals and outputting' them.
2. Description of Related Art
In recent years, mobile communication apparatus typically represented by communication systems such as GSM (Global System for Mobile Communication), PCS (Personal Communication system), PDC (Personal Digital Cellular) system and CDMA (Code Division Multiple Access) system (so-called hand set telephone) have been popularized world wide.
Generally, a mobile communication apparatus comprises an antenna for emitting and receiving electromagnetic waves, a high frequency power amplifier for amplifying high frequency signals under power modulation and supplying them to the antenna, a receiving section for processing high frequency signals received on the antenna, a control section for controlling them and a battery for supplying a power source voltage to them.
Such mobile communication apparatus and semiconductor devices used for the mobile communication apparatus have been disclosed, for example, in the known literatures described below.    (1) The constitution of the mobile communication apparatus is disclosed, for example, in “HITACHI HYORON” vol. 78, No. 11 (1996-11), pages 21 to 26 (Literature 1).    (2) Constitution of a typical high frequency power amplifier on the GSM system is described, for example, in ISSCC 98, DIGEST OF TECHNICAL PAPERS (Feb. 5, 1998), pages 50 to 55 (Literature 2).
According to the literatures, a threshold voltage of FET is controlled to an appropriate level for stable circuit design and decrease of leak current in an off state. In the constitution of the amplifier, two chips are arranged in parallel for final stage devices in a 3-stage amplifier circuit and synthesizing and an impedance-matching circuit is disposed to each of them, for the synthesis of outputs of attain higher output than in the case of using one chip. The constitution of this amplifier is referred to as DD-DIMA technique (Divided Device and Collectively Impedance Matched Amplifier) in the literature.    (3) Amplifying devices applied to the high frequency power amplifiers are described, for example, in IEDM 97 Technical Digest (1997), pages 51 to 54 (Literature 3)
This literature discloses that an amplifier device is constituted with a power insulated gate type field effect transistor (hereinafter simply referred to as power MOSFET) using Si (silicon) semiconductors to attain high performance.
Specifically, the performance is improved by defining the gate length of a MOSFET to 0.4 μm. Further, the drain breakdown voltage is set to 20 V or higher by disposing an offset layer of an about 0.7 μm length on the side of the drain of the power MOSFET. Further, it is important to lower the gate resistance for high frequency operation and the gate resistance is decreased by a structure of short circuiting a metal silicide/silicon laminated gate electrode with an aluminum wiring (Al-shorted silicon gate structure).    (4) There has been a trend of adopting compound semiconductor (GaAs) wafers for making the device efficiency higher. Such a technical trend is described, for example, in NIKKEI ELECTRONICS 1998, 11, 2 (No. 729), pages 238 to 245 (Literature 4). However, as described also in this literature, the unit wafer cost in the GaAs technique is more expensive compared with Si.
For generalized use of the mobile communication apparatus, it has been demanded for further reduction of the size and weight, and decrease in the power consumption of the apparatus. For this purpose, it is necessary to attain further reduction in the size and weight and decrease in the power consumption for each of components constituting the mobile communication apparatus.
One of the components is a high frequency power amplifier for supplying high frequency signals to the antenna. Generally, the high frequency power amplifier most consumes electric power and it is effective of saving the consumption power of the mobile apparatus to decrease the consumption power of the high frequency power amplifier (improvement of efficiency).
In the GSM system amplifier using silicon (Si) semiconductors, it was attained an output voltage of about 3.5 W and a overall efficiency (η all) of about 50% at a working frequency of 900 MHz and a power source voltage of 3.5 V. The overall efficiency is an efficiency in a high frequency power amplifier constituted with a three stage power amplifier of power MOSFET (high frequency module).
The performance of the power MOSFET using Si as the output stage amplifier device is about 55% power of a power-added efficiency (η add) at 2 W output based on the DD-CIMA technique, and it was necessary to attain a power added efficiency of 65% or more in power MOSFET in order to improve the overall efficiency of the amplifier to 55% or more.
The power added efficiency (η add) in the microwave power MOSFET is defined, for example, in “Optical Microwave Semiconductor Applied Technology” Feb. 29, 1996, first edition, first print (Published from Science Forum Co.), pages 59 to 66 (Literature 5).
Also in the PCS system amplifier, an output voltage of 2 W and an overall efficiency of 45% at a working frequency of 1900 MHz have been attained. The performance of the power MOSFET as the output stage amplifier device is about 50% at 1 W output. For improving the overall efficiency of the amplifier to 50% or more, it was necessary to attain the power-added efficiency of 55% or higher in the power MOSFET.
For improving the power-added efficiency of the amplifier device (power MOSFET), it is considered to decrease the on-resistance, gate resistance and parasitic capacitance and improvement of the mutual conductance.